Satellite broadcasting and satellite communication require antennas having high gains. Such high gains are made possible through sharp directivities, and such directivities have been considered to be possible only with such antennas as parabolic antennas. However, in order to receive radio wave signals from a satellite 36,000 km above the equator, the parabolic antennas have to have large surface areas, and they are required to be directed exactly to the satellite. Therefore, large dishes are required to ensure large surface areas, and large mechanical structures are required to keep the antennas stationary even when they are subjected to strong winds. Furthermore, they must be installed so as to be exactly directed to the satellite. For these reasons, various difficulties arise when such antennas are to be installed at homes.
Recently, there have been proposed various planar antennas using a large number of antenna elements on a single plane. From electromagnetic view point, such planar antennas are equivalent to parabolic antennas. However, according to such an antenna, its major beam is perpendicular to its major surface and, if it is simply mounted flat on a vertical wall, its beam is directed horizontally. It is therefore desired to tilt the main beam by the elevation angle of a satellite in view of ease of mounting the antenna, but such attempts have not been successful due to various problems involved in fabrication. Furthermore, a planar antenna comprises a large number of antenna elements, and a considerable loss is inevitable in collecting signals from the antenna elements. As antennas for radar, waveguide slot antennas are widely used but are too expensive for consumer use.
The theories for coaxial feeder lines have been known from the past, and have been applied to various products. The inventor is not aware of any attempt to produce a beam antenna by opening a large number of slots each having a length for resonance in a coaxial transmission line and slanted by a suitable angle relative to the longitudinal axis of the coaxial transmission line. If such an attempt were made in low frequency ranges far below the cutoff frequency of a particular coaxial cable where such coaxial cables are typically used, the length of the slots would become so long that they become spiral, and such an antenna would be quite unusable. Further, it has been common to use a waveguide and it has been inconceivable to use a coaxial cable in certain high frequency ranges.
For instance, when 12 GHz is selected for a satellite broadcast frequency, its space wave length will be .lambda..sub.0 =25 mm, and the resonance length of the slot will be .lambda..sub.0 /2=12.5 mm (in reality the resonance length will be slightly shorter than this). As it is possible to conduct 12 GHz radio wave signal with a coaxial cable whose outer conductor has an inner diameter of 10 mm (or an inner circumferential length of 31.4 mm), it is possible to form a slot antenna with this coaxial cable by opening slots having a length in the order of 10 mm at desired interval. Such coaxial cables using outer conductors which are approximately 10 mm in inner diameter are commercially available for use in VHF and UHF frequency bands. They are also used for CATV because of their favorable handling.
Since the outer conductors have small thicknesses and the underlying insulators serve as a support for cutting slots out of the outer conductor, fabrication of such a slot antenna is extremely simple. This slot antenna has the additional advantage of economy because the coaxial cables are being mass produced, and are inexpensive.
A waveguide has a higher transmission efficiency than a coaxial cable in high frequency ranges for satellite broadcasting and radar, but the transmission efficiency is not a significant problem when a coaxial cable is used as a slot antenna as its length is quite small, and the use of a coaxial cable offers advantages of economy and simplicity which far outweigh a slight loss in transmission efficiency.
As there had been no attempt to use a coaxial cable in frequency ranges near its cutoff frequency, various potential problems existed, but, since handling of high frequency signals with coaxial cables has been common in the field of measuring instruments, there were no insurmountable problems. However, it should be understood that the use of a coaxial cable is solely based on commercial availability and economy, and that forming a coaxial transmission line by rolling sheet metal is also included in the concept of the present invention.
Such a coaxial slot antenna can be used as an individual antenna, but may also be used as a primary radiation source to increase its aperture area and, hence, its gain.
It is extremely difficult to aim a high directivity antenna to a satellite which is not visible to naked eyes. However, since this slot antenna may be fabricated so as to have a directivity having a proper angle of elevation when it is mounted on a vertical wall, all that is required in installing this antenna is to adjust its azimuth angle or its bearing. This is a significant advantage over other antennas which require adjustment of both the elevation angle and the azimuth angle on installing them.
A similar slot antenna is used for telephone communication with trains (refer to Japanese patent publication No. 58-21849), but, as this antenna is intended only for short-distance communication, the length of the slots are far shorter than the resonance length and composition of directivity or polarization property of the transmitted radio wave is not considered to be important.